The Tamron 28-105mm F2.8 is special lens, it's 3.75x zoom factor at a constant F2.8 is the highest ever made for a DSLR. I was excited to have this one but the excitement was soon gone: the seller didn't mention the lens was worn out. The zoom mechanism has quite some play and the sagging of the lens caused huge coma. Only at about F7 the lens was sharp, more than 2 stops in. The coating of the front lens element also has seen better days, incorrect cleaning made a lot of small scratches. This only affect the contrast and would have been tolerable. A pretty expensive bummer as the seller basically admitted scamming me by stating the lens was in good condition beforehand (I explicitly asked about the condition of the lens) and afterwards saying "well that's the risk of buying second hand" and that I shouldn't nag about it.
About half a year later the same lens was offered but this one was explicitly offered in broken condition. The metal mount broke away from the plastic when the lens was dropped. This also damaged the rear element. Fortunately the zoom mechanism was not worn out and as extra the front element had negligible coating damage. I bought it as well with the idea of swapping components. A job that takes a lot of time and concentration.

A closer look

The sagging of the lens caused a huge coma. This sagging is caused by wornout zoom tracks in the different sleeves the slide over eachother and by wornout pins that connect the sleeves with eachother.

One of the pins that connect the sleeves

This is one of those pins The brass part has a plastic o-ring that slides through the track that is visible. This part of the lens has a metal track which wasn't worn out badly, mostly the pin. The crucial track that caries the whole weight of the lens however was made of plastic. It was worn out by 0.2mm which is desastrous for keeping the lens elements aligned.
Repairing a wornout track however is not possible. The pins travel repeatedly and with a bit of force through the track. Adding any material to the track to counter the wear will be swept away. The only theoretical option is to file the tracks to a larger diameter in a controlled way and to manufacture new pins on a lathe. All with only microns of tolerance. Since I don't have a lathe this is simply not possible even if I wanted to put a lot of time and work in it.

This leaves one option: using a donor lens that has zoom tracks in good shape

All pins are rotated 90 degrees in the teardown part, this rotates the wornout sections away and takes away a bit of the play in the lens, leaving mostly the play of the worn-out tracks.

The complete teardown

I have few images of the process but this image is taken halfway. Both lenses are taken apart completely and layed out parallel. The assembly goes in reverse but crosses over at 2 location. The rear lens element is kept, the zoom mechanism with all the sleeves is swapped and the zoom ring, electronics and mount pieces are kept.

An aweful lot of small parts makes this a 3D jigsaw.

The internal elements of the lens are shielded from dust by plastic bags. Once assembled any internal dust would require again a full teardown to be removed.

The result is that my lens now has zoom tracks that have normal wear with a low amount of play. Still a bit but the sharpness at F2.8 is workable at most focal length and good enough on F4. At F4 the last sign of residual coma are also gone.

And of course the other lens I now have is in terrible condition. Both front and rear lens elements have minor coating damage and the coma is substantial, at F5.6 the image are still not usable and only F8 and higher is sharp. The aperture ring is glued fixed and the mount is probably not parallel which messes up the focal plane.